(Reuters) - Prior to the start of the Copa America, United States coach Juergen Klinsmann had set the semi-finals as a target but will now need a new goal as the hosts prepare to take on top-ranked Argentina for spot in the final.

After an opening 2-0 loss to Colombia a last-four appearance seemed a lofty objective but Klinsmann's squad responded with wins against Costa Rica, Paraguay and Ecuador to set up a clash against the South American giants in Houston.

The United States will enjoy home-field advantage on Tuesday but that is likely to be the only thing in their favor as they take on the world's top-ranked team led by the world's best player in Lionel Messi.

"This is a special moment. This is a once in a lifetime opportunity," Klinsmann said in a report on the U.S. Soccer website. "You got into the semi-final, you made yourself proud, now go for more."

Reaching the Copa final would be the greatest achievement of Klinsmann's five-year tenure with the United States and quiet speculation about his future with the team.

Argentina, world champions in 1978 and 1986, will be determined to return home with the Copa silverware for the first time since 1993.

Certainly all the pressure will be on Argentina, who are the only team in the tournament to win all four of their matches, including a ruthless 4-1 quarter-final win against Venezuela.

If the U.S. are to have any chance of reaching the June 26th final they will have to find a way of containing Messi, who is one goal shy of becoming Argentina's all-time top scorer.

Messi, who turns 29 on Friday, has a trophy case packed with individual and club honors but the Barcelona man is still seeking his first international trophy in Argentine colors.

Back to full fitness after starting the tournament coming off the bench due to back problems, Messi did not start a match until the quarters but has four goals and trails only Chile's Eduardo Vargas (six) for the tournament's Golden Boot.

While Messi is the undisputed headliner for Argentina, the team are far from a one-man show.

The U.S. will have plenty of bases to cover facing a lineup that includes Messi's Barcelona teammate Javier Mascherano and the Manchester City duo of defender Nicolas Otamendi and forward Sergio Agüero, who was second in Premier League scoring last season.

"We don't want to make (Argentina) out to be mission impossible,” said U.S. midfielder Michael Bradley.

"At the end of the day, whoever is sharper, whoever has more guys who can compete at a high level and understand the moment, that's typically which team has a better chance to win."

(Reporting by Steve Keating in Toronto; Editing by Frank Pingue)